#include <stdbool.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <linux/err.h>
#include <linux/btf.h>
#include <elf.h>
#include <libelf.h>
#include <fcntl.h>
#include "libbpf.h"
#include "btf.h"
#include "libbpf_internal.h"
#include "strset.h"

#define BTF_EXTERN_SEC ".extern"

struct src_sec
{
    const char *sec_name;
    /* positional (not necessarily ELF) index in an array of sections */
    int id;
    /* positional (not necessarily ELF) index of a matching section in a final object file */
    int dst_id;
    /* section data offset in a matching output section */
    int dst_off;
    /* whether section is omitted from the final ELF file */
    bool skipped;
    /* whether section is an ephemeral section, not mapped to an ELF section */
    bool ephemeral;

    /* ELF info */
    size_t sec_idx;
    Elf_Scn *scn;
    Elf64_Shdr *shdr;
    Elf_Data *data;

    /* corresponding BTF DATASEC type ID */
    int sec_type_id;
};

struct src_obj
{
    const char *filename;
    int fd;
    Elf *elf;
    /* Section header strings section index */
    size_t shstrs_sec_idx;
    /* SYMTAB section index */
    size_t symtab_sec_idx;

    struct btf *btf;
    struct btf_ext *btf_ext;

    /* List of sections (including ephemeral). Slot zero is unused. */
    struct src_sec *secs;
    int sec_cnt;

    /* mapping of symbol indices from src to dst ELF */
    int *sym_map;
    /* mapping from the src BTF type IDs to dst ones */
    int *btf_type_map;
};

/* single .BTF.ext data section */
struct btf_ext_sec_data
{
    size_t rec_cnt;
    __u32 rec_sz;
    void *recs;
};

struct glob_sym
{
    /* ELF symbol index */
    int sym_idx;
    /* associated section id for .ksyms, .kconfig, etc, but not .extern */
    int sec_id;
    /* extern name offset in STRTAB */
    int name_off;
    /* optional associated BTF type ID */
    int btf_id;
    /* BTF type ID to which VAR/FUNC type is pointing to; used for
     * rewriting types when extern VAR/FUNC is resolved to a concrete
     * definition
     */
    int underlying_btf_id;
    /* sec_var index in the corresponding dst_sec, if exists */
    int var_idx;

    /* extern or resolved/global symbol */
    bool is_extern;
    /* weak or strong symbol, never goes back from strong to weak */
    bool is_weak;
};

struct dst_sec
{
    char *sec_name;
    /* positional (not necessarily ELF) index in an array of sections */
    int id;

    bool ephemeral;

    /* ELF info */
    size_t sec_idx;
    Elf_Scn *scn;
    Elf64_Shdr *shdr;
    Elf_Data *data;

    /* final output section size */
    int sec_sz;
    /* final output contents of the section */
    void *raw_data;

    /* corresponding STT_SECTION symbol index in SYMTAB */
    int sec_sym_idx;

    /* section's DATASEC variable info, emitted on BTF finalization */
    bool has_btf;
    int sec_var_cnt;
    struct btf_var_secinfo *sec_vars;

    /* section's .BTF.ext data */
    struct btf_ext_sec_data func_info;
    struct btf_ext_sec_data line_info;
    struct btf_ext_sec_data core_relo_info;
};

struct bpf_linker
{
    char *filename;
    int fd;
    Elf *elf;
    Elf64_Ehdr *elf_hdr;

    /* Output sections metadata */
    struct dst_sec *secs;
    int sec_cnt;

    struct strset *strtab_strs; /* STRTAB unique strings */
    size_t strtab_sec_idx;      /* STRTAB section index */
    size_t symtab_sec_idx;      /* SYMTAB section index */

    struct btf *btf;
    struct btf_ext *btf_ext;

    /* global (including extern) ELF symbols */
    int glob_sym_cnt;
    struct glob_sym *glob_syms;
};

#define pr_warn_elf(fmt, ...) \
    libbpf_print(LIBBPF_WARN, "libbpf: " fmt ": %s\n", ##__VA_ARGS__, elf_errmsg(-1))

static int init_output_elf(struct bpf_linker *linker, const char *file);

static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
                                const struct bpf_linker_file_opts *opts,
                                struct src_obj *obj);
static int linker_sanity_check_elf(struct src_obj *obj);
static int linker_sanity_check_elf_symtab(struct src_obj *obj, struct src_sec *sec);
static int linker_sanity_check_elf_relos(struct src_obj *obj, struct src_sec *sec);
static int linker_sanity_check_btf(struct src_obj *obj);
static int linker_sanity_check_btf_ext(struct src_obj *obj);
static int linker_fixup_btf(struct src_obj *obj);
static int linker_append_sec_data(struct bpf_linker *linker, struct src_obj *obj);
static int linker_append_elf_syms(struct bpf_linker *linker, struct src_obj *obj);
static int linker_append_elf_sym(struct bpf_linker *linker, struct src_obj *obj,
                                 Elf64_Sym *sym, const char *sym_name, int src_sym_idx);
static int linker_append_elf_relos(struct bpf_linker *linker, struct src_obj *obj);
static int linker_append_btf(struct bpf_linker *linker, struct src_obj *obj);
static int linker_append_btf_ext(struct bpf_linker *linker, struct src_obj *obj);

static int finalize_btf(struct bpf_linker *linker);
static int finalize_btf_ext(struct bpf_linker *linker);

void bpf_linker__free(struct bpf_linker *linker)
{
    int i;

    if (!linker)
        return;

    free(linker->filename);

    if (linker->elf)
        elf_end(linker->elf);

    if (linker->fd >= 0)
        close(linker->fd);

    strset__free(linker->strtab_strs);

    btf__free(linker->btf);
    btf_ext__free(linker->btf_ext);

    for (i = 1; i < linker->sec_cnt; i++)
    {
        struct dst_sec *sec = &linker->secs[i];

        free(sec->sec_name);
        free(sec->raw_data);
        free(sec->sec_vars);

        free(sec->func_info.recs);
        free(sec->line_info.recs);
        free(sec->core_relo_info.recs);
    }
    free(linker->secs);

    free(linker->glob_syms);
    free(linker);
}

struct bpf_linker *bpf_linker__new(const char *filename, struct bpf_linker_opts *opts)
{
    struct bpf_linker *linker;
    int err;

    if (!OPTS_VALID(opts, bpf_linker_opts))
        return errno = EINVAL, NULL;

    if (elf_version(EV_CURRENT) == EV_NONE)
    {
        pr_warn_elf("libelf initialization failed");
        return errno = EINVAL, NULL;
    }

    linker = calloc(1, sizeof(*linker));
    if (!linker)
        return errno = ENOMEM, NULL;

    linker->fd = -1;

    err = init_output_elf(linker, filename);
    if (err)
        goto err_out;

    return linker;

err_out:
    bpf_linker__free(linker);
    return errno = -err, NULL;
}
static struct dst_sec *add_dst_sec(struct bpf_linker *linker, const char *sec_name)
{
    struct dst_sec *secs = linker->secs, *sec;
    size_t new_cnt = linker->sec_cnt ? linker->sec_cnt + 1 : 2;

    secs = libbpf_reallocarray(secs, new_cnt, sizeof(*secs));
    if (!secs)
        return NULL;

    /* zero out newly allocated memory */
    memset(secs + linker->sec_cnt, 0, (new_cnt - linker->sec_cnt) * sizeof(*secs));

    linker->secs = secs;
    linker->sec_cnt = new_cnt;

    sec = &linker->secs[new_cnt - 1];
    sec->id = new_cnt - 1;
    sec->sec_name = strdup(sec_name);
    if (!sec->sec_name)
        return NULL;

    return sec;
}

static Elf64_Sym *add_new_sym(struct bpf_linker *linker, size_t *sym_idx)
{
    struct dst_sec *symtab = &linker->secs[linker->symtab_sec_idx];
    Elf64_Sym *syms, *sym;
    size_t sym_cnt = symtab->sec_sz / sizeof(*sym);

    syms = libbpf_reallocarray(symtab->raw_data, sym_cnt + 1, sizeof(*sym));
    if (!syms)
        return NULL;

    sym = &syms[sym_cnt];
    memset(sym, 0, sizeof(*sym));

    symtab->raw_data = syms;
    symtab->sec_sz += sizeof(*sym);
    symtab->shdr->sh_size += sizeof(*sym);
    symtab->data->d_size += sizeof(*sym);

    if (sym_idx)
        *sym_idx = sym_cnt;

    return sym;
}

static int init_output_elf(struct bpf_linker *linker, const char *file)
{
    int err, str_off;
    Elf64_Sym *init_sym;
    struct dst_sec *sec;

    linker->filename = strdup(file);
    if (!linker->filename)
        return -ENOMEM;

    linker->fd = open(file, O_WRONLY | O_CREAT | O_TRUNC | O_CLOEXEC, 0644);
    if (linker->fd < 0)
    {
        err = -errno;
        pr_warn("failed to create '%s': %d\n", file, err);
        return err;
    }

    linker->elf = elf_begin(linker->fd, ELF_C_WRITE, NULL);
    if (!linker->elf)
    {
        pr_warn_elf("failed to create ELF object");
        return -EINVAL;
    }

    /* ELF header */
    linker->elf_hdr = elf64_newehdr(linker->elf);
    if (!linker->elf_hdr)
    {
        pr_warn_elf("failed to create ELF header");
        return -EINVAL;
    }

    linker->elf_hdr->e_machine = EM_BPF;
    linker->elf_hdr->e_type = ET_REL;
#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
    linker->elf_hdr->e_ident[EI_DATA] = ELFDATA2LSB;
#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
    linker->elf_hdr->e_ident[EI_DATA] = ELFDATA2MSB;
#else
#error "Unknown __BYTE_ORDER__"
#endif

    /* STRTAB */
    /* initialize strset with an empty string to conform to ELF */
    linker->strtab_strs = strset__new(INT_MAX, "", sizeof(""));
    if (libbpf_get_error(linker->strtab_strs))
        return libbpf_get_error(linker->strtab_strs);

    sec = add_dst_sec(linker, ".strtab");
    if (!sec)
        return -ENOMEM;

    sec->scn = elf_newscn(linker->elf);
    if (!sec->scn)
    {
        pr_warn_elf("failed to create STRTAB section");
        return -EINVAL;
    }

    sec->shdr = elf64_getshdr(sec->scn);
    if (!sec->shdr)
        return -EINVAL;

    sec->data = elf_newdata(sec->scn);
    if (!sec->data)
    {
        pr_warn_elf("failed to create STRTAB data");
        return -EINVAL;
    }

    str_off = strset__add_str(linker->strtab_strs, sec->sec_name);
    if (str_off < 0)
        return str_off;

    sec->sec_idx = elf_ndxscn(sec->scn);
    linker->elf_hdr->e_shstrndx = sec->sec_idx;
    linker->strtab_sec_idx = sec->sec_idx;

    sec->shdr->sh_name = str_off;
    sec->shdr->sh_type = SHT_STRTAB;
    sec->shdr->sh_flags = SHF_STRINGS;
    sec->shdr->sh_offset = 0;
    sec->shdr->sh_link = 0;
    sec->shdr->sh_info = 0;
    sec->shdr->sh_addralign = 1;
    sec->shdr->sh_size = sec->sec_sz = 0;
    sec->shdr->sh_entsize = 0;

    /* SYMTAB */
    sec = add_dst_sec(linker, ".symtab");
    if (!sec)
        return -ENOMEM;

    sec->scn = elf_newscn(linker->elf);
    if (!sec->scn)
    {
        pr_warn_elf("failed to create SYMTAB section");
        return -EINVAL;
    }

    sec->shdr = elf64_getshdr(sec->scn);
    if (!sec->shdr)
        return -EINVAL;

    sec->data = elf_newdata(sec->scn);
    if (!sec->data)
    {
        pr_warn_elf("failed to create SYMTAB data");
        return -EINVAL;
    }

    str_off = strset__add_str(linker->strtab_strs, sec->sec_name);
    if (str_off < 0)
        return str_off;

    sec->sec_idx = elf_ndxscn(sec->scn);
    linker->symtab_sec_idx = sec->sec_idx;

    sec->shdr->sh_name = str_off;
    sec->shdr->sh_type = SHT_SYMTAB;
    sec->shdr->sh_flags = 0;
    sec->shdr->sh_offset = 0;
    sec->shdr->sh_link = linker->strtab_sec_idx;
    /* sh_info should be one greater than the index of the last local
     * symbol (i.e., binding is STB_LOCAL). But why and who cares?
     */
    sec->shdr->sh_info = 0;
    sec->shdr->sh_addralign = 8;
    sec->shdr->sh_entsize = sizeof(Elf64_Sym);

    /* .BTF */
    linker->btf = btf__new_empty();
    err = libbpf_get_error(linker->btf);
    if (err)
        return err;

    /* add the special all-zero symbol */
    init_sym = add_new_sym(linker, NULL);
    if (!init_sym)
        return -EINVAL;

    init_sym->st_name = 0;
    init_sym->st_info = 0;
    init_sym->st_other = 0;
    init_sym->st_shndx = SHN_UNDEF;
    init_sym->st_value = 0;
    init_sym->st_size = 0;

    return 0;
}
int bpf_linker__add_file(struct bpf_linker *linker, const char *filename,
                         const struct bpf_linker_file_opts *opts)
{
    struct src_obj obj = {};
    int err = 0;

    if (!OPTS_VALID(opts, bpf_linker_file_opts))
        return libbpf_err(-EINVAL);

    if (!linker->elf)
        return libbpf_err(-EINVAL);

    err = err ?: linker_load_obj_file(linker, filename, opts, &obj);
    err = err ?: linker_append_sec_data(linker, &obj);
    err = err ?: linker_append_elf_syms(linker, &obj);
    err = err ?: linker_append_elf_relos(linker, &obj);
    err = err ?: linker_append_btf(linker, &obj);
    err = err ?: linker_append_btf_ext(linker, &obj);

    /* free up src_obj resources */
    free(obj.btf_type_map);
    btf__free(obj.btf);
    btf_ext__free(obj.btf_ext);
    free(obj.secs);
    free(obj.sym_map);
    if (obj.elf)
        elf_end(obj.elf);
    if (obj.fd >= 0)
        close(obj.fd);

    return libbpf_err(err);
}

static bool is_dwarf_sec_name(const char *name)
{
    /* approximation, but the actual list is too long */
    return strncmp(name, ".debug_", sizeof(".debug_") - 1) == 0;
}

static bool is_ignored_sec(struct src_sec *sec)
{
    Elf64_Shdr *shdr = sec->shdr;
    const char *name = sec->sec_name;

    /* no special handling of .strtab */
    if (shdr->sh_type == SHT_STRTAB)
        return true;

    /* ignore .llvm_addrsig section as well */
    if (shdr->sh_type == SHT_LLVM_ADDRSIG)
        return true;

    /* no subprograms will lead to an empty .text section, ignore it */
    if (shdr->sh_type == SHT_PROGBITS && shdr->sh_size == 0 &&
        strcmp(sec->sec_name, ".text") == 0)
        return true;

    /* DWARF sections */
    if (is_dwarf_sec_name(sec->sec_name))
        return true;

    if (strncmp(name, ".rel", sizeof(".rel") - 1) == 0)
    {
        name += sizeof(".rel") - 1;
        /* DWARF section relocations */
        if (is_dwarf_sec_name(name))
            return true;

        /* .BTF and .BTF.ext don't need relocations */
        if (strcmp(name, BTF_ELF_SEC) == 0 ||
            strcmp(name, BTF_EXT_ELF_SEC) == 0)
            return true;
    }

    return false;
}

static struct src_sec *add_src_sec(struct src_obj *obj, const char *sec_name)
{
    struct src_sec *secs = obj->secs, *sec;
    size_t new_cnt = obj->sec_cnt ? obj->sec_cnt + 1 : 2;

    secs = libbpf_reallocarray(secs, new_cnt, sizeof(*secs));
    if (!secs)
        return NULL;

    /* zero out newly allocated memory */
    memset(secs + obj->sec_cnt, 0, (new_cnt - obj->sec_cnt) * sizeof(*secs));

    obj->secs = secs;
    obj->sec_cnt = new_cnt;

    sec = &obj->secs[new_cnt - 1];
    sec->id = new_cnt - 1;
    sec->sec_name = sec_name;

    return sec;
}

static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
                                const struct bpf_linker_file_opts *opts,
                                struct src_obj *obj)
{
#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
    const int host_endianness = ELFDATA2LSB;
#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
    const int host_endianness = ELFDATA2MSB;
#else
#error "Unknown __BYTE_ORDER__"
#endif
    int err = 0;
    Elf_Scn *scn;
    Elf_Data *data;
    Elf64_Ehdr *ehdr;
    Elf64_Shdr *shdr;
    struct src_sec *sec;

    pr_debug("linker: adding object file '%s'...\n", filename);

    obj->filename = filename;

    obj->fd = open(filename, O_RDONLY | O_CLOEXEC);
    if (obj->fd < 0)
    {
        err = -errno;
        pr_warn("failed to open file '%s': %d\n", filename, err);
        return err;
    }
    obj->elf = elf_begin(obj->fd, ELF_C_READ_MMAP, NULL);
    if (!obj->elf)
    {
        err = -errno;
        pr_warn_elf("failed to parse ELF file '%s'", filename);
        return err;
    }

    /* Sanity check ELF file high-level properties */
    ehdr = elf64_getehdr(obj->elf);
    if (!ehdr)
    {
        err = -errno;
        pr_warn_elf("failed to get ELF header for %s", filename);
        return err;
    }
    if (ehdr->e_ident[EI_DATA] != host_endianness)
    {
        err = -EOPNOTSUPP;
        pr_warn_elf("unsupported byte order of ELF file %s", filename);
        return err;
    }
    if (ehdr->e_type != ET_REL || ehdr->e_machine != EM_BPF || ehdr->e_ident[EI_CLASS] != ELFCLASS64)
    {
        err = -EOPNOTSUPP;
        pr_warn_elf("unsupported kind of ELF file %s", filename);
        return err;
    }

    if (elf_getshdrstrndx(obj->elf, &obj->shstrs_sec_idx))
    {
        err = -errno;
        pr_warn_elf("failed to get SHSTRTAB section index for %s", filename);
        return err;
    }

    scn = NULL;
    while ((scn = elf_nextscn(obj->elf, scn)) != NULL)
    {
        size_t sec_idx = elf_ndxscn(scn);
        const char *sec_name;

        shdr = elf64_getshdr(scn);
        if (!shdr)
        {
            err = -errno;
            pr_warn_elf("failed to get section #%zu header for %s",
                        sec_idx, filename);
            return err;
        }

        sec_name = elf_strptr(obj->elf, obj->shstrs_sec_idx, shdr->sh_name);
        if (!sec_name)
        {
            err = -errno;
            pr_warn_elf("failed to get section #%zu name for %s",
                        sec_idx, filename);
            return err;
        }

        data = elf_getdata(scn, 0);
        if (!data)
        {
            err = -errno;
            pr_warn_elf("failed to get section #%zu (%s) data from %s",
                        sec_idx, sec_name, filename);
            return err;
        }

        sec = add_src_sec(obj, sec_name);
        if (!sec)
            return -ENOMEM;

        sec->scn = scn;
        sec->shdr = shdr;
        sec->data = data;
        sec->sec_idx = elf_ndxscn(scn);

        if (is_ignored_sec(sec))
        {
            sec->skipped = true;
            continue;
        }

        switch (shdr->sh_type)
        {
        case SHT_SYMTAB:
            if (obj->symtab_sec_idx)
            {
                err = -EOPNOTSUPP;
                pr_warn("multiple SYMTAB sections found, not supported\n");
                return err;
            }
            obj->symtab_sec_idx = sec_idx;
            break;
        case SHT_STRTAB:
            /* we'll construct our own string table */
            break;
        case SHT_PROGBITS:
            if (strcmp(sec_name, BTF_ELF_SEC) == 0)
            {
                obj->btf = btf__new(data->d_buf, shdr->sh_size);
                err = libbpf_get_error(obj->btf);
                if (err)
                {
                    pr_warn("failed to parse .BTF from %s: %d\n", filename, err);
                    return err;
                }
                sec->skipped = true;
                continue;
            }
            if (strcmp(sec_name, BTF_EXT_ELF_SEC) == 0)
            {
                obj->btf_ext = btf_ext__new(data->d_buf, shdr->sh_size);
                err = libbpf_get_error(obj->btf_ext);
                if (err)
                {
                    pr_warn("failed to parse .BTF.ext from '%s': %d\n", filename, err);
                    return err;
                }
                sec->skipped = true;
                continue;
            }

            /* data & code */
            break;
        case SHT_NOBITS:
            /* BSS */
            break;
        case SHT_REL:
            /* relocations */
            break;
        default:
            pr_warn("unrecognized section #%zu (%s) in %s\n",
                    sec_idx, sec_name, filename);
            err = -EINVAL;
            return err;
        }
    }

    err = err ?: linker_sanity_check_elf(obj);
    err = err ?: linker_sanity_check_btf(obj);
    err = err ?: linker_sanity_check_btf_ext(obj);
    err = err ?: linker_fixup_btf(obj);

    return err;
}
static int linker_sanity_check_elf(struct src_obj *obj)
{
    struct src_sec *sec;
    int i, err;

    if (!obj->symtab_sec_idx)
    {
        pr_warn("ELF is missing SYMTAB section in %s\n", obj->filename);
        return -EINVAL;
    }
    if (!obj->shstrs_sec_idx)
    {
        pr_warn("ELF is missing section headers STRTAB section in %s\n", obj->filename);
        return -EINVAL;
    }

    for (i = 1; i < obj->sec_cnt; i++)
    {
        sec = &obj->secs[i];

        if (sec->sec_name[0] == '\0')
        {
            pr_warn("ELF section #%zu has empty name in %s\n", sec->sec_idx, obj->filename);
            return -EINVAL;
        }

        if (sec->shdr->sh_addralign && !is_pow_of_2(sec->shdr->sh_addralign))
            return -EINVAL;
        if (sec->shdr->sh_addralign != sec->data->d_align)
            return -EINVAL;

        if (sec->shdr->sh_size != sec->data->d_size)
            return -EINVAL;

        switch (sec->shdr->sh_type)
        {
        case SHT_SYMTAB:
            err = linker_sanity_check_elf_symtab(obj, sec);
            if (err)
                return err;
            break;
        case SHT_STRTAB:
            break;
        case SHT_PROGBITS:
            if (sec->shdr->sh_flags & SHF_EXECINSTR)
            {
                if (sec->shdr->sh_size % sizeof(struct bpf_insn) != 0)
                    return -EINVAL;
            }
            break;
        case SHT_NOBITS:
            break;
        case SHT_REL:
            err = linker_sanity_check_elf_relos(obj, sec);
            if (err)
                return err;
            break;
        case SHT_LLVM_ADDRSIG:
            break;
        default:
            pr_warn("ELF section #%zu (%s) has unrecognized type %zu in %s\n",
                    sec->sec_idx, sec->sec_name, (size_t)sec->shdr->sh_type, obj->filename);
            return -EINVAL;
        }
    }

    return 0;
}

static int linker_sanity_check_elf_symtab(struct src_obj *obj, struct src_sec *sec)
{
    struct src_sec *link_sec;
    Elf64_Sym *sym;
    int i, n;

    if (sec->shdr->sh_entsize != sizeof(Elf64_Sym))
        return -EINVAL;
    if (sec->shdr->sh_size % sec->shdr->sh_entsize != 0)
        return -EINVAL;

    if (!sec->shdr->sh_link || sec->shdr->sh_link >= obj->sec_cnt)
    {
        pr_warn("ELF SYMTAB section #%zu points to missing STRTAB section #%zu in %s\n",
                sec->sec_idx, (size_t)sec->shdr->sh_link, obj->filename);
        return -EINVAL;
    }
    link_sec = &obj->secs[sec->shdr->sh_link];
    if (link_sec->shdr->sh_type != SHT_STRTAB)
    {
        pr_warn("ELF SYMTAB section #%zu points to invalid STRTAB section #%zu in %s\n",
                sec->sec_idx, (size_t)sec->shdr->sh_link, obj->filename);
        return -EINVAL;
    }

    n = sec->shdr->sh_size / sec->shdr->sh_entsize;
    sym = sec->data->d_buf;
    for (i = 0; i < n; i++, sym++)
    {
        int sym_type = ELF64_ST_TYPE(sym->st_info);
        int sym_bind = ELF64_ST_BIND(sym->st_info);
        int sym_vis = ELF64_ST_VISIBILITY(sym->st_other);

        if (i == 0)
        {
            if (sym->st_name != 0 || sym->st_info != 0 || sym->st_other != 0 || sym->st_shndx != 0 || sym->st_value != 0 || sym->st_size != 0)
            {
                pr_warn("ELF sym #0 is invalid in %s\n", obj->filename);
                return -EINVAL;
            }
            continue;
        }
        if (sym_bind != STB_LOCAL && sym_bind != STB_GLOBAL && sym_bind != STB_WEAK)
        {
            pr_warn("ELF sym #%d in section #%zu has unsupported symbol binding %d\n",
                    i, sec->sec_idx, sym_bind);
            return -EINVAL;
        }
        if (sym_vis != STV_DEFAULT && sym_vis != STV_HIDDEN)
        {
            pr_warn("ELF sym #%d in section #%zu has unsupported symbol visibility %d\n",
                    i, sec->sec_idx, sym_vis);
            return -EINVAL;
        }
        if (sym->st_shndx == 0)
        {
            if (sym_type != STT_NOTYPE || sym_bind == STB_LOCAL || sym->st_value != 0 || sym->st_size != 0)
            {
                pr_warn("ELF sym #%d is invalid extern symbol in %s\n",
                        i, obj->filename);

                return -EINVAL;
            }
            continue;
        }
        if (sym->st_shndx < SHN_LORESERVE && sym->st_shndx >= obj->sec_cnt)
        {
            pr_warn("ELF sym #%d in section #%zu points to missing section #%zu in %s\n",
                    i, sec->sec_idx, (size_t)sym->st_shndx, obj->filename);
            return -EINVAL;
        }
        if (sym_type == STT_SECTION)
        {
            if (sym->st_value != 0)
                return -EINVAL;
            continue;
        }
    }

    return 0;
}

static int linker_sanity_check_elf_relos(struct src_obj *obj, struct src_sec *sec)
{
    struct src_sec *link_sec, *sym_sec;
    Elf64_Rel *relo;
    int i, n;

    if (sec->shdr->sh_entsize != sizeof(Elf64_Rel))
        return -EINVAL;
    if (sec->shdr->sh_size % sec->shdr->sh_entsize != 0)
        return -EINVAL;

    /* SHT_REL's sh_link should point to SYMTAB */
    if (sec->shdr->sh_link != obj->symtab_sec_idx)
    {
        pr_warn("ELF relo section #%zu points to invalid SYMTAB section #%zu in %s\n",
                sec->sec_idx, (size_t)sec->shdr->sh_link, obj->filename);
        return -EINVAL;
    }

    /* SHT_REL's sh_info points to relocated section */
    if (!sec->shdr->sh_info || sec->shdr->sh_info >= obj->sec_cnt)
    {
        pr_warn("ELF relo section #%zu points to missing section #%zu in %s\n",
                sec->sec_idx, (size_t)sec->shdr->sh_info, obj->filename);
        return -EINVAL;
    }
    link_sec = &obj->secs[sec->shdr->sh_info];

    /* .rel<secname> -> <secname> pattern is followed */
    if (strncmp(sec->sec_name, ".rel", sizeof(".rel") - 1) != 0 || strcmp(sec->sec_name + sizeof(".rel") - 1, link_sec->sec_name) != 0)
    {
        pr_warn("ELF relo section #%zu name has invalid name in %s\n",
                sec->sec_idx, obj->filename);
        return -EINVAL;
    }

    /* don't further validate relocations for ignored sections */
    if (link_sec->skipped)
        return 0;

    /* relocatable section is data or instructions */
    if (link_sec->shdr->sh_type != SHT_PROGBITS && link_sec->shdr->sh_type != SHT_NOBITS)
    {
        pr_warn("ELF relo section #%zu points to invalid section #%zu in %s\n",
                sec->sec_idx, (size_t)sec->shdr->sh_info, obj->filename);
        return -EINVAL;
    }

    /* check sanity of each relocation */
    n = sec->shdr->sh_size / sec->shdr->sh_entsize;
    relo = sec->data->d_buf;
    sym_sec = &obj->secs[obj->symtab_sec_idx];
    for (i = 0; i < n; i++, relo++)
    {
        size_t sym_idx = ELF64_R_SYM(relo->r_info);
        size_t sym_type = ELF64_R_TYPE(relo->r_info);

        if (sym_type != R_BPF_64_64 && sym_type != R_BPF_64_32 &&
            sym_type != R_BPF_64_ABS64 && sym_type != R_BPF_64_ABS32)
        {
            pr_warn("ELF relo #%d in section #%zu has unexpected type %zu in %s\n",
                    i, sec->sec_idx, sym_type, obj->filename);
            return -EINVAL;
        }

        if (!sym_idx || sym_idx * sizeof(Elf64_Sym) >= sym_sec->shdr->sh_size)
        {
            pr_warn("ELF relo #%d in section #%zu points to invalid symbol #%zu in %s\n",
                    i, sec->sec_idx, sym_idx, obj->filename);
            return -EINVAL;
        }

        if (link_sec->shdr->sh_flags & SHF_EXECINSTR)
        {
            if (relo->r_offset % sizeof(struct bpf_insn) != 0)
            {
                pr_warn("ELF relo #%d in section #%zu points to missing symbol #%zu in %s\n",
                        i, sec->sec_idx, sym_idx, obj->filename);
                return -EINVAL;
            }
        }
    }

    return 0;
}

static int check_btf_type_id(__u32 *type_id, void *ctx)
{
    struct btf *btf = ctx;

    if (*type_id >= btf__type_cnt(btf))
        return -EINVAL;

    return 0;
}

static int check_btf_str_off(__u32 *str_off, void *ctx)
{
    struct btf *btf = ctx;
    const char *s;

    s = btf__str_by_offset(btf, *str_off);

    if (!s)
        return -EINVAL;

    return 0;
}

static int linker_sanity_check_btf(struct src_obj *obj)
{
    struct btf_type *t;
    int i, n, err = 0;

    if (!obj->btf)
        return 0;

    n = btf__type_cnt(obj->btf);
    for (i = 1; i < n; i++)
    {
        t = btf_type_by_id(obj->btf, i);

        err = err ?: btf_type_visit_type_ids(t, check_btf_type_id, obj->btf);
        err = err ?: btf_type_visit_str_offs(t, check_btf_str_off, obj->btf);
        if (err)
            return err;
    }

    return 0;
}

static int linker_sanity_check_btf_ext(struct src_obj *obj)
{
    int err = 0;

    if (!obj->btf_ext)
        return 0;

    /* can't use .BTF.ext without .BTF */
    if (!obj->btf)
        return -EINVAL;

    err = err ?: btf_ext_visit_type_ids(obj->btf_ext, check_btf_type_id, obj->btf);
    err = err ?: btf_ext_visit_str_offs(obj->btf_ext, check_btf_str_off, obj->btf);
    if (err)
        return err;

    return 0;
}
static int init_sec(struct bpf_linker *linker, struct dst_sec *dst_sec, struct src_sec *src_sec)
{
    Elf_Scn *scn;
    Elf_Data *data;
    Elf64_Shdr *shdr;
    int name_off;

    dst_sec->sec_sz = 0;
    dst_sec->sec_idx = 0;
    dst_sec->ephemeral = src_sec->ephemeral;

    /* ephemeral sections are just thin section shells lacking most parts */
    if (src_sec->ephemeral)
        return 0;

    scn = elf_newscn(linker->elf);
    if (!scn)
        return -ENOMEM;
    data = elf_newdata(scn);
    if (!data)
        return -ENOMEM;
    shdr = elf64_getshdr(scn);
    if (!shdr)
        return -ENOMEM;

    dst_sec->scn = scn;
    dst_sec->shdr = shdr;
    dst_sec->data = data;
    dst_sec->sec_idx = elf_ndxscn(scn);

    name_off = strset__add_str(linker->strtab_strs, src_sec->sec_name);
    if (name_off < 0)
        return name_off;

    shdr->sh_name = name_off;
    shdr->sh_type = src_sec->shdr->sh_type;
    shdr->sh_flags = src_sec->shdr->sh_flags;
    shdr->sh_size = 0;
    /* sh_link and sh_info have different meaning for different types of
     * sections, so we leave it up to the caller code to fill them in, if
     * necessary
     */
    shdr->sh_link = 0;
    shdr->sh_info = 0;
    shdr->sh_addralign = src_sec->shdr->sh_addralign;
    shdr->sh_entsize = src_sec->shdr->sh_entsize;

    data->d_type = src_sec->data->d_type;
    data->d_size = 0;
    data->d_buf = NULL;
    data->d_align = src_sec->data->d_align;
    data->d_off = 0;

    return 0;
}
static struct dst_sec *find_dst_sec_by_name(struct bpf_linker *linker, const char *sec_name)
{
    struct dst_sec *sec;
    int i;

    for (i = 1; i < linker->sec_cnt; i++)
    {
        sec = &linker->secs[i];

        if (strcmp(sec->sec_name, sec_name) == 0)
            return sec;
    }

    return NULL;
}

static bool secs_match(struct dst_sec *dst, struct src_sec *src)
{
    if (dst->ephemeral || src->ephemeral)
        return true;

    if (dst->shdr->sh_type != src->shdr->sh_type)
    {
        pr_warn("sec %s types mismatch\n", dst->sec_name);
        return false;
    }
    if (dst->shdr->sh_flags != src->shdr->sh_flags)
    {
        pr_warn("sec %s flags mismatch\n", dst->sec_name);
        return false;
    }
    if (dst->shdr->sh_entsize != src->shdr->sh_entsize)
    {
        pr_warn("sec %s entsize mismatch\n", dst->sec_name);
        return false;
    }

    return true;
}

static bool sec_content_is_same(struct dst_sec *dst_sec, struct src_sec *src_sec)
{
    if (dst_sec->sec_sz != src_sec->shdr->sh_size)
        return false;
    if (memcmp(dst_sec->raw_data, src_sec->data->d_buf, dst_sec->sec_sz) != 0)
        return false;
    return true;
}
static int extend_sec(struct bpf_linker *linker, struct dst_sec *dst, struct src_sec *src)
{
    void *tmp;
    size_t dst_align, src_align;
    size_t dst_align_sz, dst_final_sz;
    int err;

    if (src->ephemeral)
        return 0;

    if (dst->ephemeral)
    {
        err = init_sec(linker, dst, src);
        if (err)
            return err;
    }

    dst_align = dst->shdr->sh_addralign;
    src_align = src->shdr->sh_addralign;
    if (dst_align == 0)
        dst_align = 1;
    if (dst_align < src_align)
        dst_align = src_align;

    dst_align_sz = (dst->sec_sz + dst_align - 1) / dst_align * dst_align;

    dst_final_sz = dst_align_sz + src->shdr->sh_size;

    if (src->shdr->sh_type != SHT_NOBITS)
    {
        tmp = realloc(dst->raw_data, dst_final_sz);
        if (!tmp && dst_align_sz > 0)
            return -ENOMEM;
        dst->raw_data = tmp;

        memset(dst->raw_data + dst->sec_sz, 0, dst_align_sz - dst->sec_sz);
        memcpy(dst->raw_data + dst_align_sz, src->data->d_buf, src->shdr->sh_size);
    }

    dst->sec_sz = dst_final_sz;
    dst->shdr->sh_size = dst_final_sz;
    dst->data->d_size = dst_final_sz;

    dst->shdr->sh_addralign = dst_align;
    dst->data->d_align = dst_align;

    src->dst_off = dst_align_sz;

    return 0;
}

static bool is_data_sec(struct src_sec *sec)
{
    if (!sec || sec->skipped)
        return false;
    if (sec->ephemeral)
        return true;
    return sec->shdr->sh_type == SHT_PROGBITS || sec->shdr->sh_type == SHT_NOBITS;
}

static bool is_relo_sec(struct src_sec *sec)
{
    if (!sec || sec->skipped || sec->ephemeral)
        return false;
    return sec->shdr->sh_type == SHT_REL;
}

static int linker_append_sec_data(struct bpf_linker *linker, struct src_obj *obj)
{
    int i, err;

    for (i = 1; i < obj->sec_cnt; i++)
    {
        struct src_sec *src_sec;
        struct dst_sec *dst_sec;

        src_sec = &obj->secs[i];
        if (!is_data_sec(src_sec))
            continue;

        dst_sec = find_dst_sec_by_name(linker, src_sec->sec_name);
        if (!dst_sec)
        {
            dst_sec = add_dst_sec(linker, src_sec->sec_name);
            if (!dst_sec)
                return -ENOMEM;
            err = init_sec(linker, dst_sec, src_sec);
            if (err)
            {
                pr_warn("failed to init section '%s'\n", src_sec->sec_name);
                return err;
            }
        }
        else
        {
            if (!secs_match(dst_sec, src_sec))
            {
                pr_warn("ELF sections %s are incompatible\n", src_sec->sec_name);
                return -1;
            }

            /* "license" and "version" sections are deduped */
            if (strcmp(src_sec->sec_name, "license") == 0 || strcmp(src_sec->sec_name, "version") == 0)
            {
                if (!sec_content_is_same(dst_sec, src_sec))
                {
                    pr_warn("non-identical contents of section '%s' are not supported\n", src_sec->sec_name);
                    return -EINVAL;
                }
                src_sec->skipped = true;
                src_sec->dst_id = dst_sec->id;
                continue;
            }
        }

        /* record mapped section index */
        src_sec->dst_id = dst_sec->id;

        err = extend_sec(linker, dst_sec, src_sec);
        if (err)
            return err;
    }

    return 0;
}
static int linker_append_elf_syms(struct bpf_linker *linker, struct src_obj *obj)
{
    struct src_sec *symtab = &obj->secs[obj->symtab_sec_idx];
    Elf64_Sym *sym = symtab->data->d_buf;
    int i, n = symtab->shdr->sh_size / symtab->shdr->sh_entsize, err;
    int str_sec_idx = symtab->shdr->sh_link;
    const char *sym_name;

    obj->sym_map = calloc(n + 1, sizeof(*obj->sym_map));
    if (!obj->sym_map)
        return -ENOMEM;

    for (i = 0; i < n; i++, sym++)
    {
        /* We already validated all-zero symbol #0 and we already
         * appended it preventively to the final SYMTAB, so skip it.
         */
        if (i == 0)
            continue;

        sym_name = elf_strptr(obj->elf, str_sec_idx, sym->st_name);
        if (!sym_name)
        {
            pr_warn("can't fetch symbol name for symbol #%d in '%s'\n", i, obj->filename);
            return -EINVAL;
        }

        err = linker_append_elf_sym(linker, obj, sym, sym_name, i);
        if (err)
            return err;
    }

    return 0;
}

static Elf64_Sym *get_sym_by_idx(struct bpf_linker *linker, size_t sym_idx)
{
    struct dst_sec *symtab = &linker->secs[linker->symtab_sec_idx];
    Elf64_Sym *syms = symtab->raw_data;

    return &syms[sym_idx];
}

static struct glob_sym *find_glob_sym(struct bpf_linker *linker, const char *sym_name)
{
    struct glob_sym *glob_sym;
    const char *name;
    int i;

    for (i = 0; i < linker->glob_sym_cnt; i++)
    {
        glob_sym = &linker->glob_syms[i];
        name = strset__data(linker->strtab_strs) + glob_sym->name_off;

        if (strcmp(name, sym_name) == 0)
            return glob_sym;
    }

    return NULL;
}

static struct glob_sym *add_glob_sym(struct bpf_linker *linker)
{
    struct glob_sym *syms, *sym;

    syms = libbpf_reallocarray(linker->glob_syms, linker->glob_sym_cnt + 1,
                               sizeof(*linker->glob_syms));
    if (!syms)
        return NULL;

    sym = &syms[linker->glob_sym_cnt];
    memset(sym, 0, sizeof(*sym));
    sym->var_idx = -1;

    linker->glob_syms = syms;
    linker->glob_sym_cnt++;

    return sym;
}

static bool glob_sym_btf_matches(const char *sym_name, bool exact,
                                 const struct btf *btf1, __u32 id1,
                                 const struct btf *btf2, __u32 id2)
{
    const struct btf_type *t1, *t2;
    bool is_static1, is_static2;
    const char *n1, *n2;
    int i, n;

recur:
    n1 = n2 = NULL;
    t1 = skip_mods_and_typedefs(btf1, id1, &id1);
    t2 = skip_mods_and_typedefs(btf2, id2, &id2);

    /* check if only one side is FWD, otherwise handle with common logic */
    if (!exact && btf_is_fwd(t1) != btf_is_fwd(t2))
    {
        n1 = btf__str_by_offset(btf1, t1->name_off);
        n2 = btf__str_by_offset(btf2, t2->name_off);
        if (strcmp(n1, n2) != 0)
        {
            pr_warn("global '%s': incompatible forward declaration names '%s' and '%s'\n",
                    sym_name, n1, n2);
            return false;
        }
        /* validate if FWD kind matches concrete kind */
        if (btf_is_fwd(t1))
        {
            if (btf_kflag(t1) && btf_is_union(t2))
                return true;
            if (!btf_kflag(t1) && btf_is_struct(t2))
                return true;
            pr_warn("global '%s': incompatible %s forward declaration and concrete kind %s\n",
                    sym_name, btf_kflag(t1) ? "union" : "struct", btf_kind_str(t2));
        }
        else
        {
            if (btf_kflag(t2) && btf_is_union(t1))
                return true;
            if (!btf_kflag(t2) && btf_is_struct(t1))
                return true;
            pr_warn("global '%s': incompatible %s forward declaration and concrete kind %s\n",
                    sym_name, btf_kflag(t2) ? "union" : "struct", btf_kind_str(t1));
        }
        return false;
    }

    if (btf_kind(t1) != btf_kind(t2))
    {
        pr_warn("global '%s': incompatible BTF kinds %s and %s\n",
                sym_name, btf_kind_str(t1), btf_kind_str(t2));
        return false;
    }

    switch (btf_kind(t1))
    {
    case BTF_KIND_STRUCT:
    case BTF_KIND_UNION:
    case BTF_KIND_ENUM:
    case BTF_KIND_ENUM64:
    case BTF_KIND_FWD:
    case BTF_KIND_FUNC:
    case BTF_KIND_VAR:
        n1 = btf__str_by_offset(btf1, t1->name_off);
        n2 = btf__str_by_offset(btf2, t2->name_off);
        if (strcmp(n1, n2) != 0)
        {
            pr_warn("global '%s': incompatible %s names '%s' and '%s'\n",
                    sym_name, btf_kind_str(t1), n1, n2);
            return false;
        }
        break;
    default:
        break;
    }

    switch (btf_kind(t1))
    {
    case BTF_KIND_UNKN: /* void */
    case BTF_KIND_FWD:
        return true;
    case BTF_KIND_INT:
    case BTF_KIND_FLOAT:
    case BTF_KIND_ENUM:
    case BTF_KIND_ENUM64:
        /* ignore encoding for int and enum values for enum */
        if (t1->size != t2->size)
        {
            pr_warn("global '%s': incompatible %s '%s' size %u and %u\n",
                    sym_name, btf_kind_str(t1), n1, t1->size, t2->size);
            return false;
        }
        return true;
    case BTF_KIND_PTR:
        /* just validate overall shape of the referenced type, so no
         * contents comparison for struct/union, and allowd fwd vs
         * struct/union
         */
        exact = false;
        id1 = t1->type;
        id2 = t2->type;
        goto recur;
    case BTF_KIND_ARRAY:
        /* ignore index type and array size */
        id1 = btf_array(t1)->type;
        id2 = btf_array(t2)->type;
        goto recur;
    case BTF_KIND_FUNC:
        /* extern and global linkages are compatible */
        is_static1 = btf_func_linkage(t1) == BTF_FUNC_STATIC;
        is_static2 = btf_func_linkage(t2) == BTF_FUNC_STATIC;
        if (is_static1 != is_static2)
        {
            pr_warn("global '%s': incompatible func '%s' linkage\n", sym_name, n1);
            return false;
        }

        id1 = t1->type;
        id2 = t2->type;
        goto recur;
    case BTF_KIND_VAR:
        /* extern and global linkages are compatible */
        is_static1 = btf_var(t1)->linkage == BTF_VAR_STATIC;
        is_static2 = btf_var(t2)->linkage == BTF_VAR_STATIC;
        if (is_static1 != is_static2)
        {
            pr_warn("global '%s': incompatible var '%s' linkage\n", sym_name, n1);
            return false;
        }

        id1 = t1->type;
        id2 = t2->type;
        goto recur;
    case BTF_KIND_STRUCT:
    case BTF_KIND_UNION:
    {
        const struct btf_member *m1, *m2;

        if (!exact)
            return true;

        if (btf_vlen(t1) != btf_vlen(t2))
        {
            pr_warn("global '%s': incompatible number of %s fields %u and %u\n",
                    sym_name, btf_kind_str(t1), btf_vlen(t1), btf_vlen(t2));
            return false;
        }

        n = btf_vlen(t1);
        m1 = btf_members(t1);
        m2 = btf_members(t2);
        for (i = 0; i < n; i++, m1++, m2++)
        {
            n1 = btf__str_by_offset(btf1, m1->name_off);
            n2 = btf__str_by_offset(btf2, m2->name_off);
            if (strcmp(n1, n2) != 0)
            {
                pr_warn("global '%s': incompatible field #%d names '%s' and '%s'\n",
                        sym_name, i, n1, n2);
                return false;
            }
            if (m1->offset != m2->offset)
            {
                pr_warn("global '%s': incompatible field #%d ('%s') offsets\n",
                        sym_name, i, n1);
                return false;
            }
            if (!glob_sym_btf_matches(sym_name, exact, btf1, m1->type, btf2, m2->type))
                return false;
        }

        return true;
    }
    case BTF_KIND_FUNC_PROTO:
    {
        const struct btf_param *m1, *m2;

        if (btf_vlen(t1) != btf_vlen(t2))
        {
            pr_warn("global '%s': incompatible number of %s params %u and %u\n",
                    sym_name, btf_kind_str(t1), btf_vlen(t1), btf_vlen(t2));
            return false;
        }

        n = btf_vlen(t1);
        m1 = btf_params(t1);
        m2 = btf_params(t2);
        for (i = 0; i < n; i++, m1++, m2++)
        {
            /* ignore func arg names */
            if (!glob_sym_btf_matches(sym_name, exact, btf1, m1->type, btf2, m2->type))
                return false;
        }

        /* now check return type as well */
        id1 = t1->type;
        id2 = t2->type;
        goto recur;
    }

    /* skip_mods_and_typedefs() make this impossible */
    case BTF_KIND_TYPEDEF:
    case BTF_KIND_VOLATILE:
    case BTF_KIND_CONST:
    case BTF_KIND_RESTRICT:
    /* DATASECs are never compared with each other */
    case BTF_KIND_DATASEC:
    default:
        pr_warn("global '%s': unsupported BTF kind %s\n",
                sym_name, btf_kind_str(t1));
        return false;
    }
}

static bool map_defs_match(const char *sym_name,
                           const struct btf *main_btf,
                           const struct btf_map_def *main_def,
                           const struct btf_map_def *main_inner_def,
                           const struct btf *extra_btf,
                           const struct btf_map_def *extra_def,
                           const struct btf_map_def *extra_inner_def)
{
    const char *reason;

    if (main_def->map_type != extra_def->map_type)
    {
        reason = "type";
        goto mismatch;
    }

    /* check key type/size match */
    if (main_def->key_size != extra_def->key_size)
    {
        reason = "key_size";
        goto mismatch;
    }
    if (!!main_def->key_type_id != !!extra_def->key_type_id)
    {
        reason = "key type";
        goto mismatch;
    }
    if ((main_def->parts & MAP_DEF_KEY_TYPE) && !glob_sym_btf_matches(sym_name, true /*exact*/,
                                                                      main_btf, main_def->key_type_id,
                                                                      extra_btf, extra_def->key_type_id))
    {
        reason = "key type";
        goto mismatch;
    }

    /* validate value type/size match */
    if (main_def->value_size != extra_def->value_size)
    {
        reason = "value_size";
        goto mismatch;
    }
    if (!!main_def->value_type_id != !!extra_def->value_type_id)
    {
        reason = "value type";
        goto mismatch;
    }
    if ((main_def->parts & MAP_DEF_VALUE_TYPE) && !glob_sym_btf_matches(sym_name, true /*exact*/,
                                                                        main_btf, main_def->value_type_id,
                                                                        extra_btf, extra_def->value_type_id))
    {
        reason = "key type";
        goto mismatch;
    }

    if (main_def->max_entries != extra_def->max_entries)
    {
        reason = "max_entries";
        goto mismatch;
    }
    if (main_def->map_flags != extra_def->map_flags)
    {
        reason = "map_flags";
        goto mismatch;
    }
    if (main_def->numa_node != extra_def->numa_node)
    {
        reason = "numa_node";
        goto mismatch;
    }
    if (main_def->pinning != extra_def->pinning)
    {
        reason = "pinning";
        goto mismatch;
    }

    if ((main_def->parts & MAP_DEF_INNER_MAP) != (extra_def->parts & MAP_DEF_INNER_MAP))
    {
        reason = "inner map";
        goto mismatch;
    }

    if (main_def->parts & MAP_DEF_INNER_MAP)
    {
        char inner_map_name[128];

        snprintf(inner_map_name, sizeof(inner_map_name), "%s.inner", sym_name);

        return map_defs_match(inner_map_name,
                              main_btf, main_inner_def, NULL,
                              extra_btf, extra_inner_def, NULL);
    }

    return true;

mismatch:
    pr_warn("global '%s': map %s mismatch\n", sym_name, reason);
    return false;
}

static bool glob_map_defs_match(const char *sym_name,
                                struct bpf_linker *linker, struct glob_sym *glob_sym,
                                struct src_obj *obj, Elf64_Sym *sym, int btf_id)
{
    struct btf_map_def dst_def = {}, dst_inner_def = {};
    struct btf_map_def src_def = {}, src_inner_def = {};
    const struct btf_type *t;
    int err;

    t = btf__type_by_id(obj->btf, btf_id);
    if (!btf_is_var(t))
    {
        pr_warn("global '%s': invalid map definition type [%d]\n", sym_name, btf_id);
        return false;
    }
    t = skip_mods_and_typedefs(obj->btf, t->type, NULL);

    err = parse_btf_map_def(sym_name, obj->btf, t, true /*strict*/, &src_def, &src_inner_def);
    if (err)
    {
        pr_warn("global '%s': invalid map definition\n", sym_name);
        return false;
    }

    /* re-parse existing map definition */
    t = btf__type_by_id(linker->btf, glob_sym->btf_id);
    t = skip_mods_and_typedefs(linker->btf, t->type, NULL);
    err = parse_btf_map_def(sym_name, linker->btf, t, true /*strict*/, &dst_def, &dst_inner_def);
    if (err)
    {
        /* this should not happen, because we already validated it */
        pr_warn("global '%s': invalid dst map definition\n", sym_name);
        return false;
    }

    /* Currently extern map definition has to be complete and match
     * concrete map definition exactly. This restriction might be lifted
     * in the future.
     */
    return map_defs_match(sym_name, linker->btf, &dst_def, &dst_inner_def,
                          obj->btf, &src_def, &src_inner_def);
}

static bool glob_syms_match(const char *sym_name,
                            struct bpf_linker *linker, struct glob_sym *glob_sym,
                            struct src_obj *obj, Elf64_Sym *sym, size_t sym_idx, int btf_id)
{
    const struct btf_type *src_t;

    /* if we are dealing with externs, BTF types describing both global
     * and extern VARs/FUNCs should be completely present in all files
     */
    if (!glob_sym->btf_id || !btf_id)
    {
        pr_warn("BTF info is missing for global symbol '%s'\n", sym_name);
        return false;
    }

    src_t = btf__type_by_id(obj->btf, btf_id);
    if (!btf_is_var(src_t) && !btf_is_func(src_t))
    {
        pr_warn("only extern variables and functions are supported, but got '%s' for '%s'\n",
                btf_kind_str(src_t), sym_name);
        return false;
    }

    /* deal with .maps definitions specially */
    if (glob_sym->sec_id && strcmp(linker->secs[glob_sym->sec_id].sec_name, MAPS_ELF_SEC) == 0)
        return glob_map_defs_match(sym_name, linker, glob_sym, obj, sym, btf_id);

    if (!glob_sym_btf_matches(sym_name, true /*exact*/,
                              linker->btf, glob_sym->btf_id, obj->btf, btf_id))
        return false;

    return true;
}

static bool btf_is_non_static(const struct btf_type *t)
{
    return (btf_is_var(t) && btf_var(t)->linkage != BTF_VAR_STATIC) || (btf_is_func(t) && btf_func_linkage(t) != BTF_FUNC_STATIC);
}

static int find_glob_sym_btf(struct src_obj *obj, Elf64_Sym *sym, const char *sym_name,
                             int *out_btf_sec_id, int *out_btf_id)
{
    int i, j, n, m, btf_id = 0;
    const struct btf_type *t;
    const struct btf_var_secinfo *vi;
    const char *name;

    if (!obj->btf)
    {
        pr_warn("failed to find BTF info for object '%s'\n", obj->filename);
        return -EINVAL;
    }

    n = btf__type_cnt(obj->btf);
    for (i = 1; i < n; i++)
    {
        t = btf__type_by_id(obj->btf, i);

        /* some global and extern FUNCs and VARs might not be associated with any
         * DATASEC, so try to detect them in the same pass
         */
        if (btf_is_non_static(t))
        {
            name = btf__str_by_offset(obj->btf, t->name_off);
            if (strcmp(name, sym_name) != 0)
                continue;

            /* remember and still try to find DATASEC */
            btf_id = i;
            continue;
        }

        if (!btf_is_datasec(t))
            continue;

        vi = btf_var_secinfos(t);
        for (j = 0, m = btf_vlen(t); j < m; j++, vi++)
        {
            t = btf__type_by_id(obj->btf, vi->type);
            name = btf__str_by_offset(obj->btf, t->name_off);

            if (strcmp(name, sym_name) != 0)
                continue;
            if (btf_is_var(t) && btf_var(t)->linkage == BTF_VAR_STATIC)
                continue;
            if (btf_is_func(t) && btf_func_linkage(t) == BTF_FUNC_STATIC)
                continue;

            if (btf_id && btf_id != vi->type)
            {
                pr_warn("global/extern '%s' BTF is ambiguous: both types #%d and #%u match\n",
                        sym_name, btf_id, vi->type);
                return -EINVAL;
            }

            *out_btf_sec_id = i;
            *out_btf_id = vi->type;

            return 0;
        }
    }

    /* free-floating extern or global FUNC */
    if (btf_id)
    {
        *out_btf_sec_id = 0;
        *out_btf_id = btf_id;
        return 0;
    }

    pr_warn("failed to find BTF info for global/extern symbol '%s'\n", sym_name);
    return -ENOENT;
}

static struct src_sec *find_src_sec_by_name(struct src_obj *obj, const char *sec_name)
{
    struct src_sec *sec;
    int i;

    for (i = 1; i < obj->sec_cnt; i++)
    {
        sec = &obj->secs[i];

        if (strcmp(sec->sec_name, sec_name) == 0)
            return sec;
    }

    return NULL;
}

static int complete_extern_btf_info(struct btf *dst_btf, int dst_id,
                                    struct btf *src_btf, int src_id)
{
    struct btf_type *dst_t = btf_type_by_id(dst_btf, dst_id);
    struct btf_type *src_t = btf_type_by_id(src_btf, src_id);
    struct btf_param *src_p, *dst_p;
    const char *s;
    int i, n, off;

    if (btf_is_var(dst_t))
    {
        btf_var(dst_t)->linkage = BTF_VAR_GLOBAL_ALLOCATED;
        return 0;
    }

    dst_t->info = btf_type_info(BTF_KIND_FUNC, BTF_FUNC_GLOBAL, 0);

    /* now onto FUNC_PROTO types */
    src_t = btf_type_by_id(src_btf, src_t->type);
    dst_t = btf_type_by_id(dst_btf, dst_t->type);

    src_p = btf_params(src_t);
    dst_p = btf_params(dst_t);
    for (i = 0, n = btf_vlen(dst_t); i < n; i++, src_p++, dst_p++)
    {
        if (!src_p->name_off)
            continue;

        /* src_btf has more complete info, so add name to dst_btf */
        s = btf__str_by_offset(src_btf, src_p->name_off);
        off = btf__add_str(dst_btf, s);
        if (off < 0)
            return off;
        dst_p->name_off = off;
    }
    return 0;
}
static void sym_update_bind(Elf64_Sym *sym, int sym_bind)
{
    sym->st_info = ELF64_ST_INFO(sym_bind, ELF64_ST_TYPE(sym->st_info));
}

static void sym_update_type(Elf64_Sym *sym, int sym_type)
{
    sym->st_info = ELF64_ST_INFO(ELF64_ST_BIND(sym->st_info), sym_type);
}

static void sym_update_visibility(Elf64_Sym *sym, int sym_vis)
{
    /* libelf doesn't provide setters for ST_VISIBILITY,
     * but it is stored in the lower 2 bits of st_other
     */
    sym->st_other &= ~0x03;
    sym->st_other |= sym_vis;
}

static int linker_append_elf_sym(struct bpf_linker *linker, struct src_obj *obj,
                                 Elf64_Sym *sym, const char *sym_name, int src_sym_idx)
{
    struct src_sec *src_sec = NULL;
    struct dst_sec *dst_sec = NULL;
    struct glob_sym *glob_sym = NULL;
    int name_off, sym_type, sym_bind, sym_vis, err;
    int btf_sec_id = 0, btf_id = 0;
    size_t dst_sym_idx;
    Elf64_Sym *dst_sym;
    bool sym_is_extern;

    sym_type = ELF64_ST_TYPE(sym->st_info);
    sym_bind = ELF64_ST_BIND(sym->st_info);
    sym_vis = ELF64_ST_VISIBILITY(sym->st_other);
    sym_is_extern = sym->st_shndx == SHN_UNDEF;

    if (sym_is_extern)
    {
        if (!obj->btf)
        {
            pr_warn("externs without BTF info are not supported\n");
            return -ENOTSUP;
        }
    }
    else if (sym->st_shndx < SHN_LORESERVE)
    {
        src_sec = &obj->secs[sym->st_shndx];
        if (src_sec->skipped)
            return 0;
        dst_sec = &linker->secs[src_sec->dst_id];

        /* allow only one STT_SECTION symbol per section */
        if (sym_type == STT_SECTION && dst_sec->sec_sym_idx)
        {
            obj->sym_map[src_sym_idx] = dst_sec->sec_sym_idx;
            return 0;
        }
    }

    if (sym_bind == STB_LOCAL)
        goto add_sym;

    /* find matching BTF info */
    err = find_glob_sym_btf(obj, sym, sym_name, &btf_sec_id, &btf_id);
    if (err)
        return err;

    if (sym_is_extern && btf_sec_id)
    {
        const char *sec_name = NULL;
        const struct btf_type *t;

        t = btf__type_by_id(obj->btf, btf_sec_id);
        sec_name = btf__str_by_offset(obj->btf, t->name_off);

        if (strcmp(sec_name, BTF_EXTERN_SEC) != 0)
        {
            src_sec = find_src_sec_by_name(obj, sec_name);
            if (!src_sec)
            {
                pr_warn("failed to find matching ELF sec '%s'\n", sec_name);
                return -ENOENT;
            }
            dst_sec = &linker->secs[src_sec->dst_id];
        }
    }

    glob_sym = find_glob_sym(linker, sym_name);
    if (glob_sym)
    {
        obj->sym_map[src_sym_idx] = glob_sym->sym_idx;

        if (!sym_is_extern && !glob_sym->is_extern && !glob_sym->is_weak && sym_bind != STB_WEAK)
        {
            pr_warn("conflicting non-weak symbol #%d (%s) definition in '%s'\n",
                    src_sym_idx, sym_name, obj->filename);
            return -EINVAL;
        }

        if (!glob_syms_match(sym_name, linker, glob_sym, obj, sym, src_sym_idx, btf_id))
            return -EINVAL;

        dst_sym = get_sym_by_idx(linker, glob_sym->sym_idx);

        if (sym_bind == STB_GLOBAL)
        {

            sym_update_bind(dst_sym, STB_GLOBAL);
            glob_sym->is_weak = false;
        }

        if (sym_vis > ELF64_ST_VISIBILITY(dst_sym->st_other))
            sym_update_visibility(dst_sym, sym_vis);

        if (sym_is_extern)
            return 0;

        if (!glob_sym->is_extern && sym_bind == STB_WEAK)
            return 0;

        sym_update_type(dst_sym, sym_type);
        dst_sym->st_shndx = dst_sec->sec_idx;
        dst_sym->st_value = src_sec->dst_off + sym->st_value;
        dst_sym->st_size = sym->st_size;

        glob_sym->sec_id = dst_sec->id;
        glob_sym->is_extern = false;

        if (complete_extern_btf_info(linker->btf, glob_sym->btf_id,
                                     obj->btf, btf_id))
            return -EINVAL;

        glob_sym->underlying_btf_id = 0;

        obj->sym_map[src_sym_idx] = glob_sym->sym_idx;
        return 0;
    }

add_sym:
    name_off = strset__add_str(linker->strtab_strs, sym_name);
    if (name_off < 0)
        return name_off;

    dst_sym = add_new_sym(linker, &dst_sym_idx);
    if (!dst_sym)
        return -ENOMEM;

    dst_sym->st_name = name_off;
    dst_sym->st_info = sym->st_info;
    dst_sym->st_other = sym->st_other;
    dst_sym->st_shndx = dst_sec ? dst_sec->sec_idx : sym->st_shndx;
    dst_sym->st_value = (src_sec ? src_sec->dst_off : 0) + sym->st_value;
    dst_sym->st_size = sym->st_size;

    obj->sym_map[src_sym_idx] = dst_sym_idx;

    if (sym_type == STT_SECTION && dst_sym)
    {
        dst_sec->sec_sym_idx = dst_sym_idx;
        dst_sym->st_value = 0;
    }

    if (sym_bind != STB_LOCAL)
    {
        glob_sym = add_glob_sym(linker);
        if (!glob_sym)
            return -ENOMEM;

        glob_sym->sym_idx = dst_sym_idx;

        glob_sym->sec_id = dst_sec ? dst_sec->id : 0;
        glob_sym->name_off = name_off;
        glob_sym->btf_id = 0;
        glob_sym->is_extern = sym_is_extern;
        glob_sym->is_weak = sym_bind == STB_WEAK;
    }

    return 0;
}
static int linker_append_elf_relos(struct bpf_linker *linker, struct src_obj *obj)
{
    struct src_sec *src_symtab = &obj->secs[obj->symtab_sec_idx];
    int i, err;

    for (i = 1; i < obj->sec_cnt; i++)
    {
        struct src_sec *src_sec, *src_linked_sec;
        struct dst_sec *dst_sec, *dst_linked_sec;
        Elf64_Rel *src_rel, *dst_rel;
        int j, n;

        src_sec = &obj->secs[i];
        if (!is_relo_sec(src_sec))
            continue;

        /* shdr->sh_info points to relocatable section */
        src_linked_sec = &obj->secs[src_sec->shdr->sh_info];
        if (src_linked_sec->skipped)
            continue;

        dst_sec = find_dst_sec_by_name(linker, src_sec->sec_name);
        if (!dst_sec)
        {
            dst_sec = add_dst_sec(linker, src_sec->sec_name);
            if (!dst_sec)
                return -ENOMEM;
            err = init_sec(linker, dst_sec, src_sec);
            if (err)
            {
                pr_warn("failed to init section '%s'\n", src_sec->sec_name);
                return err;
            }
        }
        else if (!secs_match(dst_sec, src_sec))
        {
            pr_warn("sections %s are not compatible\n", src_sec->sec_name);
            return -1;
        }

        /* shdr->sh_link points to SYMTAB */
        dst_sec->shdr->sh_link = linker->symtab_sec_idx;

        /* shdr->sh_info points to relocated section */
        dst_linked_sec = &linker->secs[src_linked_sec->dst_id];
        dst_sec->shdr->sh_info = dst_linked_sec->sec_idx;

        src_sec->dst_id = dst_sec->id;
        err = extend_sec(linker, dst_sec, src_sec);
        if (err)
            return err;

        src_rel = src_sec->data->d_buf;
        dst_rel = dst_sec->raw_data + src_sec->dst_off;
        n = src_sec->shdr->sh_size / src_sec->shdr->sh_entsize;
        for (j = 0; j < n; j++, src_rel++, dst_rel++)
        {
            size_t src_sym_idx, dst_sym_idx, sym_type;
            Elf64_Sym *src_sym;

            src_sym_idx = ELF64_R_SYM(src_rel->r_info);
            src_sym = src_symtab->data->d_buf + sizeof(*src_sym) * src_sym_idx;

            dst_sym_idx = obj->sym_map[src_sym_idx];
            dst_rel->r_offset += src_linked_sec->dst_off;
            sym_type = ELF64_R_TYPE(src_rel->r_info);
            dst_rel->r_info = ELF64_R_INFO(dst_sym_idx, sym_type);

            if (ELF64_ST_TYPE(src_sym->st_info) == STT_SECTION)
            {
                struct src_sec *sec = &obj->secs[src_sym->st_shndx];
                struct bpf_insn *insn;

                if (src_linked_sec->shdr->sh_flags & SHF_EXECINSTR)
                {
                    insn = dst_linked_sec->raw_data + dst_rel->r_offset;
                    if (insn->code == (BPF_JMP | BPF_CALL))
                        insn->imm += sec->dst_off / sizeof(struct bpf_insn);
                    else
                        insn->imm += sec->dst_off;
                }
                else
                {
                    pr_warn("relocation against STT_SECTION in non-exec section is not supported!\n");
                    return -EINVAL;
                }
            }
        }
    }

    return 0;
}
static Elf64_Sym *find_sym_by_name(struct src_obj *obj, size_t sec_idx,
                                   int sym_type, const char *sym_name)
{
    struct src_sec *symtab = &obj->secs[obj->symtab_sec_idx];
    Elf64_Sym *sym = symtab->data->d_buf;
    int i, n = symtab->shdr->sh_size / symtab->shdr->sh_entsize;
    int str_sec_idx = symtab->shdr->sh_link;
    const char *name;

    for (i = 0; i < n; i++, sym++)
    {
        if (sym->st_shndx != sec_idx)
            continue;
        if (ELF64_ST_TYPE(sym->st_info) != sym_type)
            continue;

        name = elf_strptr(obj->elf, str_sec_idx, sym->st_name);
        if (!name)
            return NULL;

        if (strcmp(sym_name, name) != 0)
            continue;

        return sym;
    }

    return NULL;
}

static int linker_fixup_btf(struct src_obj *obj)
{
    const char *sec_name;
    struct src_sec *sec;
    int i, j, n, m;

    if (!obj->btf)
        return 0;

    n = btf__type_cnt(obj->btf);
    for (i = 1; i < n; i++)
    {
        struct btf_var_secinfo *vi;
        struct btf_type *t;

        t = btf_type_by_id(obj->btf, i);
        if (btf_kind(t) != BTF_KIND_DATASEC)
            continue;

        sec_name = btf__str_by_offset(obj->btf, t->name_off);
        sec = find_src_sec_by_name(obj, sec_name);
        if (sec)
        {
            if (sec->shdr)
                t->size = sec->shdr->sh_size;
        }
        else
        {
            if (strcmp(sec_name, BTF_EXTERN_SEC) == 0)
                continue;

            sec = add_src_sec(obj, sec_name);
            if (!sec)
                return -ENOMEM;

            sec->ephemeral = true;
            sec->sec_idx = 0; /* will match UNDEF shndx in ELF */
        }

        sec->sec_type_id = i;

        vi = btf_var_secinfos(t);
        for (j = 0, m = btf_vlen(t); j < m; j++, vi++)
        {
            const struct btf_type *vt = btf__type_by_id(obj->btf, vi->type);
            const char *var_name = btf__str_by_offset(obj->btf, vt->name_off);
            int var_linkage = btf_var(vt)->linkage;
            Elf64_Sym *sym;

            if (var_linkage != BTF_VAR_GLOBAL_ALLOCATED)
                continue;

            sym = find_sym_by_name(obj, sec->sec_idx, STT_OBJECT, var_name);
            if (!sym)
            {
                pr_warn("failed to find symbol for variable '%s' in section '%s'\n", var_name, sec_name);
                return -ENOENT;
            }

            vi->offset = sym->st_value;
        }
    }

    return 0;
}

static int remap_type_id(__u32 *type_id, void *ctx)
{
    int *id_map = ctx;
    int new_id = id_map[*type_id];

    /* Error out if the type wasn't remapped. Ignore VOID which stays VOID. */
    if (new_id == 0 && *type_id != 0)
    {
        pr_warn("failed to find new ID mapping for original BTF type ID %u\n", *type_id);
        return -EINVAL;
    }

    *type_id = id_map[*type_id];

    return 0;
}

static int linker_append_btf(struct bpf_linker *linker, struct src_obj *obj)
{
    const struct btf_type *t;
    int i, j, n, start_id, id;
    const char *name;

    if (!obj->btf)
        return 0;

    start_id = btf__type_cnt(linker->btf);
    n = btf__type_cnt(obj->btf);

    obj->btf_type_map = calloc(n + 1, sizeof(int));
    if (!obj->btf_type_map)
        return -ENOMEM;

    for (i = 1; i < n; i++)
    {
        struct glob_sym *glob_sym = NULL;

        t = btf__type_by_id(obj->btf, i);

        /* DATASECs are handled specially below */
        if (btf_kind(t) == BTF_KIND_DATASEC)
            continue;

        if (btf_is_non_static(t))
        {
            /* there should be glob_sym already */
            name = btf__str_by_offset(obj->btf, t->name_off);
            glob_sym = find_glob_sym(linker, name);
            if (!glob_sym)
                continue;

            if (glob_sym->underlying_btf_id == 0)
                glob_sym->underlying_btf_id = -t->type;

            if (glob_sym->btf_id)
            {
                /* reuse existing BTF type for global var/func */
                obj->btf_type_map[i] = glob_sym->btf_id;
                continue;
            }
        }

        id = btf__add_type(linker->btf, obj->btf, t);
        if (id < 0)
        {
            pr_warn("failed to append BTF type #%d from file '%s'\n", i, obj->filename);
            return id;
        }

        obj->btf_type_map[i] = id;

        /* record just appended BTF type for var/func */
        if (glob_sym)
        {
            glob_sym->btf_id = id;
            glob_sym->underlying_btf_id = -t->type;
        }
    }

    /* remap all the types except DATASECs */
    n = btf__type_cnt(linker->btf);
    for (i = start_id; i < n; i++)
    {
        struct btf_type *dst_t = btf_type_by_id(linker->btf, i);

        if (btf_type_visit_type_ids(dst_t, remap_type_id, obj->btf_type_map))
            return -EINVAL;
    }

    for (i = 0; i < linker->glob_sym_cnt; i++)
    {
        struct glob_sym *glob_sym = &linker->glob_syms[i];
        struct btf_type *glob_t;

        if (glob_sym->underlying_btf_id >= 0)
            continue;

        glob_sym->underlying_btf_id = obj->btf_type_map[-glob_sym->underlying_btf_id];

        glob_t = btf_type_by_id(linker->btf, glob_sym->btf_id);
        glob_t->type = glob_sym->underlying_btf_id;
    }

    /* append DATASEC info */
    for (i = 1; i < obj->sec_cnt; i++)
    {
        struct src_sec *src_sec;
        struct dst_sec *dst_sec;
        const struct btf_var_secinfo *src_var;
        struct btf_var_secinfo *dst_var;

        src_sec = &obj->secs[i];
        if (!src_sec->sec_type_id || src_sec->skipped)
            continue;
        dst_sec = &linker->secs[src_sec->dst_id];

        dst_sec->has_btf = true;

        t = btf__type_by_id(obj->btf, src_sec->sec_type_id);
        src_var = btf_var_secinfos(t);
        n = btf_vlen(t);
        for (j = 0; j < n; j++, src_var++)
        {
            void *sec_vars = dst_sec->sec_vars;
            int new_id = obj->btf_type_map[src_var->type];
            struct glob_sym *glob_sym = NULL;

            t = btf_type_by_id(linker->btf, new_id);
            if (btf_is_non_static(t))
            {
                name = btf__str_by_offset(linker->btf, t->name_off);
                glob_sym = find_glob_sym(linker, name);
                if (glob_sym->sec_id != dst_sec->id)
                {
                    pr_warn("global '%s': section mismatch %d vs %d\n",
                            name, glob_sym->sec_id, dst_sec->id);
                    return -EINVAL;
                }
            }

            if (glob_sym && glob_sym->var_idx >= 0)
            {
                __s64 sz;

                dst_var = &dst_sec->sec_vars[glob_sym->var_idx];

                sz = btf__resolve_size(linker->btf, glob_sym->underlying_btf_id);
                if (sz < 0)
                {
                    pr_warn("global '%s': failed to resolve size of underlying type: %d\n",
                            name, (int)sz);
                    return -EINVAL;
                }
                dst_var->size = sz;
                continue;
            }

            sec_vars = libbpf_reallocarray(sec_vars,
                                           dst_sec->sec_var_cnt + 1,
                                           sizeof(*dst_sec->sec_vars));
            if (!sec_vars)
                return -ENOMEM;

            dst_sec->sec_vars = sec_vars;
            dst_sec->sec_var_cnt++;

            dst_var = &dst_sec->sec_vars[dst_sec->sec_var_cnt - 1];
            dst_var->type = obj->btf_type_map[src_var->type];
            dst_var->size = src_var->size;
            dst_var->offset = src_sec->dst_off + src_var->offset;

            if (glob_sym)
                glob_sym->var_idx = dst_sec->sec_var_cnt - 1;
        }
    }

    return 0;
}

static void *add_btf_ext_rec(struct btf_ext_sec_data *ext_data, const void *src_rec)
{
    void *tmp;

    tmp = libbpf_reallocarray(ext_data->recs, ext_data->rec_cnt + 1, ext_data->rec_sz);
    if (!tmp)
        return NULL;
    ext_data->recs = tmp;

    tmp += ext_data->rec_cnt * ext_data->rec_sz;
    memcpy(tmp, src_rec, ext_data->rec_sz);

    ext_data->rec_cnt++;

    return tmp;
}

static int linker_append_btf_ext(struct bpf_linker *linker, struct src_obj *obj)
{
    const struct btf_ext_info_sec *ext_sec;
    const char *sec_name, *s;
    struct src_sec *src_sec;
    struct dst_sec *dst_sec;
    int rec_sz, str_off, i;

    if (!obj->btf_ext)
        return 0;

    rec_sz = obj->btf_ext->func_info.rec_size;
    for_each_btf_ext_sec(&obj->btf_ext->func_info, ext_sec)
    {
        struct bpf_func_info_min *src_rec, *dst_rec;

        sec_name = btf__name_by_offset(obj->btf, ext_sec->sec_name_off);
        src_sec = find_src_sec_by_name(obj, sec_name);
        if (!src_sec)
        {
            pr_warn("can't find section '%s' referenced from .BTF.ext\n", sec_name);
            return -EINVAL;
        }
        dst_sec = &linker->secs[src_sec->dst_id];

        if (dst_sec->func_info.rec_sz == 0)
            dst_sec->func_info.rec_sz = rec_sz;
        if (dst_sec->func_info.rec_sz != rec_sz)
        {
            pr_warn("incompatible .BTF.ext record sizes for section '%s'\n", sec_name);
            return -EINVAL;
        }

        for_each_btf_ext_rec(&obj->btf_ext->func_info, ext_sec, i, src_rec)
        {
            dst_rec = add_btf_ext_rec(&dst_sec->func_info, src_rec);
            if (!dst_rec)
                return -ENOMEM;

            dst_rec->insn_off += src_sec->dst_off;
            dst_rec->type_id = obj->btf_type_map[dst_rec->type_id];
        }
    }

    rec_sz = obj->btf_ext->line_info.rec_size;
    for_each_btf_ext_sec(&obj->btf_ext->line_info, ext_sec)
    {
        struct bpf_line_info_min *src_rec, *dst_rec;

        sec_name = btf__name_by_offset(obj->btf, ext_sec->sec_name_off);
        src_sec = find_src_sec_by_name(obj, sec_name);
        if (!src_sec)
        {
            pr_warn("can't find section '%s' referenced from .BTF.ext\n", sec_name);
            return -EINVAL;
        }
        dst_sec = &linker->secs[src_sec->dst_id];

        if (dst_sec->line_info.rec_sz == 0)
            dst_sec->line_info.rec_sz = rec_sz;
        if (dst_sec->line_info.rec_sz != rec_sz)
        {
            pr_warn("incompatible .BTF.ext record sizes for section '%s'\n", sec_name);
            return -EINVAL;
        }

        for_each_btf_ext_rec(&obj->btf_ext->line_info, ext_sec, i, src_rec)
        {
            dst_rec = add_btf_ext_rec(&dst_sec->line_info, src_rec);
            if (!dst_rec)
                return -ENOMEM;

            dst_rec->insn_off += src_sec->dst_off;

            s = btf__str_by_offset(obj->btf, src_rec->file_name_off);
            str_off = btf__add_str(linker->btf, s);
            if (str_off < 0)
                return -ENOMEM;
            dst_rec->file_name_off = str_off;

            s = btf__str_by_offset(obj->btf, src_rec->line_off);
            str_off = btf__add_str(linker->btf, s);
            if (str_off < 0)
                return -ENOMEM;
            dst_rec->line_off = str_off;

            /* dst_rec->line_col is fine */
        }
    }

    rec_sz = obj->btf_ext->core_relo_info.rec_size;
    for_each_btf_ext_sec(&obj->btf_ext->core_relo_info, ext_sec)
    {
        struct bpf_core_relo *src_rec, *dst_rec;

        sec_name = btf__name_by_offset(obj->btf, ext_sec->sec_name_off);
        src_sec = find_src_sec_by_name(obj, sec_name);
        if (!src_sec)
        {
            pr_warn("can't find section '%s' referenced from .BTF.ext\n", sec_name);
            return -EINVAL;
        }
        dst_sec = &linker->secs[src_sec->dst_id];

        if (dst_sec->core_relo_info.rec_sz == 0)
            dst_sec->core_relo_info.rec_sz = rec_sz;
        if (dst_sec->core_relo_info.rec_sz != rec_sz)
        {
            pr_warn("incompatible .BTF.ext record sizes for section '%s'\n", sec_name);
            return -EINVAL;
        }

        for_each_btf_ext_rec(&obj->btf_ext->core_relo_info, ext_sec, i, src_rec)
        {
            dst_rec = add_btf_ext_rec(&dst_sec->core_relo_info, src_rec);
            if (!dst_rec)
                return -ENOMEM;

            dst_rec->insn_off += src_sec->dst_off;
            dst_rec->type_id = obj->btf_type_map[dst_rec->type_id];

            s = btf__str_by_offset(obj->btf, src_rec->access_str_off);
            str_off = btf__add_str(linker->btf, s);
            if (str_off < 0)
                return -ENOMEM;
            dst_rec->access_str_off = str_off;

            /* dst_rec->kind is fine */
        }
    }

    return 0;
}

int bpf_linker__finalize(struct bpf_linker *linker)
{
    struct dst_sec *sec;
    size_t strs_sz;
    const void *strs;
    int err, i;

    if (!linker->elf)
        return libbpf_err(-EINVAL);

    err = finalize_btf(linker);
    if (err)
        return libbpf_err(err);

    /* Finalize strings */
    strs_sz = strset__data_size(linker->strtab_strs);
    strs = strset__data(linker->strtab_strs);

    sec = &linker->secs[linker->strtab_sec_idx];
    sec->data->d_align = 1;
    sec->data->d_off = 0LL;
    sec->data->d_buf = (void *)strs;
    sec->data->d_type = ELF_T_BYTE;
    sec->data->d_size = strs_sz;
    sec->shdr->sh_size = strs_sz;

    for (i = 1; i < linker->sec_cnt; i++)
    {
        sec = &linker->secs[i];

        /* STRTAB is handled specially above */
        if (sec->sec_idx == linker->strtab_sec_idx)
            continue;

        /* special ephemeral sections (.ksyms, .kconfig, etc) */
        if (!sec->scn)
            continue;

        sec->data->d_buf = sec->raw_data;
    }

    /* Finalize ELF layout */
    if (elf_update(linker->elf, ELF_C_NULL) < 0)
    {
        err = -errno;
        pr_warn_elf("failed to finalize ELF layout");
        return libbpf_err(err);
    }

    /* Write out final ELF contents */
    if (elf_update(linker->elf, ELF_C_WRITE) < 0)
    {
        err = -errno;
        pr_warn_elf("failed to write ELF contents");
        return libbpf_err(err);
    }

    elf_end(linker->elf);
    close(linker->fd);

    linker->elf = NULL;
    linker->fd = -1;

    return 0;
}

static int emit_elf_data_sec(struct bpf_linker *linker, const char *sec_name,
                             size_t align, const void *raw_data, size_t raw_sz)
{
    Elf_Scn *scn;
    Elf_Data *data;
    Elf64_Shdr *shdr;
    int name_off;

    name_off = strset__add_str(linker->strtab_strs, sec_name);
    if (name_off < 0)
        return name_off;

    scn = elf_newscn(linker->elf);
    if (!scn)
        return -ENOMEM;
    data = elf_newdata(scn);
    if (!data)
        return -ENOMEM;
    shdr = elf64_getshdr(scn);
    if (!shdr)
        return -EINVAL;

    shdr->sh_name = name_off;
    shdr->sh_type = SHT_PROGBITS;
    shdr->sh_flags = 0;
    shdr->sh_size = raw_sz;
    shdr->sh_link = 0;
    shdr->sh_info = 0;
    shdr->sh_addralign = align;
    shdr->sh_entsize = 0;

    data->d_type = ELF_T_BYTE;
    data->d_size = raw_sz;
    data->d_buf = (void *)raw_data;
    data->d_align = align;
    data->d_off = 0;

    return 0;
}

static int finalize_btf(struct bpf_linker *linker)
{
    LIBBPF_OPTS(btf_dedup_opts, opts);
    struct btf *btf = linker->btf;
    const void *raw_data;
    int i, j, id, err;
    __u32 raw_sz;

    /* bail out if no BTF data was produced */
    if (btf__type_cnt(linker->btf) == 1)
        return 0;

    for (i = 1; i < linker->sec_cnt; i++)
    {
        struct dst_sec *sec = &linker->secs[i];

        if (!sec->has_btf)
            continue;

        id = btf__add_datasec(btf, sec->sec_name, sec->sec_sz);
        if (id < 0)
        {
            pr_warn("failed to add consolidated BTF type for datasec '%s': %d\n",
                    sec->sec_name, id);
            return id;
        }

        for (j = 0; j < sec->sec_var_cnt; j++)
        {
            struct btf_var_secinfo *vi = &sec->sec_vars[j];

            if (btf__add_datasec_var_info(btf, vi->type, vi->offset, vi->size))
                return -EINVAL;
        }
    }

    err = finalize_btf_ext(linker);
    if (err)
    {
        pr_warn(".BTF.ext generation failed: %d\n", err);
        return err;
    }

    opts.btf_ext = linker->btf_ext;
    err = btf__dedup(linker->btf, &opts);
    if (err)
    {
        pr_warn("BTF dedup failed: %d\n", err);
        return err;
    }

    /* Emit .BTF section */
    raw_data = btf__raw_data(linker->btf, &raw_sz);
    if (!raw_data)
        return -ENOMEM;

    err = emit_elf_data_sec(linker, BTF_ELF_SEC, 8, raw_data, raw_sz);
    if (err)
    {
        pr_warn("failed to write out .BTF ELF section: %d\n", err);
        return err;
    }

    /* Emit .BTF.ext section */
    if (linker->btf_ext)
    {
        raw_data = btf_ext__get_raw_data(linker->btf_ext, &raw_sz);
        if (!raw_data)
            return -ENOMEM;

        err = emit_elf_data_sec(linker, BTF_EXT_ELF_SEC, 8, raw_data, raw_sz);
        if (err)
        {
            pr_warn("failed to write out .BTF.ext ELF section: %d\n", err);
            return err;
        }
    }

    return 0;
}
static int emit_btf_ext_data(struct bpf_linker *linker, void *output,
                             const char *sec_name, struct btf_ext_sec_data *sec_data)
{
    struct btf_ext_info_sec *sec_info;
    void *cur = output;
    int str_off;
    size_t sz;

    if (!sec_data->rec_cnt)
        return 0;

    str_off = btf__add_str(linker->btf, sec_name);
    if (str_off < 0)
        return -ENOMEM;

    sec_info = cur;
    sec_info->sec_name_off = str_off;
    sec_info->num_info = sec_data->rec_cnt;
    cur += sizeof(struct btf_ext_info_sec);

    sz = sec_data->rec_cnt * sec_data->rec_sz;
    memcpy(cur, sec_data->recs, sz);
    cur += sz;

    return cur - output;
}

static int finalize_btf_ext(struct bpf_linker *linker)
{
    size_t funcs_sz = 0, lines_sz = 0, core_relos_sz = 0, total_sz = 0;
    size_t func_rec_sz = 0, line_rec_sz = 0, core_relo_rec_sz = 0;
    struct btf_ext_header *hdr;
    void *data, *cur;
    int i, err, sz;

    /* validate that all sections have the same .BTF.ext record sizes
     * and calculate total data size for each type of data (func info,
     * line info, core relos)
     */
    for (i = 1; i < linker->sec_cnt; i++)
    {
        struct dst_sec *sec = &linker->secs[i];

        if (sec->func_info.rec_cnt)
        {
            if (func_rec_sz == 0)
                func_rec_sz = sec->func_info.rec_sz;
            if (func_rec_sz != sec->func_info.rec_sz)
            {
                pr_warn("mismatch in func_info record size %zu != %u\n",
                        func_rec_sz, sec->func_info.rec_sz);
                return -EINVAL;
            }

            funcs_sz += sizeof(struct btf_ext_info_sec) + func_rec_sz * sec->func_info.rec_cnt;
        }
        if (sec->line_info.rec_cnt)
        {
            if (line_rec_sz == 0)
                line_rec_sz = sec->line_info.rec_sz;
            if (line_rec_sz != sec->line_info.rec_sz)
            {
                pr_warn("mismatch in line_info record size %zu != %u\n",
                        line_rec_sz, sec->line_info.rec_sz);
                return -EINVAL;
            }

            lines_sz += sizeof(struct btf_ext_info_sec) + line_rec_sz * sec->line_info.rec_cnt;
        }
        if (sec->core_relo_info.rec_cnt)
        {
            if (core_relo_rec_sz == 0)
                core_relo_rec_sz = sec->core_relo_info.rec_sz;
            if (core_relo_rec_sz != sec->core_relo_info.rec_sz)
            {
                pr_warn("mismatch in core_relo_info record size %zu != %u\n",
                        core_relo_rec_sz, sec->core_relo_info.rec_sz);
                return -EINVAL;
            }

            core_relos_sz += sizeof(struct btf_ext_info_sec) + core_relo_rec_sz * sec->core_relo_info.rec_cnt;
        }
    }

    if (!funcs_sz && !lines_sz && !core_relos_sz)
        return 0;

    total_sz += sizeof(struct btf_ext_header);
    if (funcs_sz)
    {
        funcs_sz += sizeof(__u32); /* record size prefix */
        total_sz += funcs_sz;
    }
    if (lines_sz)
    {
        lines_sz += sizeof(__u32); /* record size prefix */
        total_sz += lines_sz;
    }
    if (core_relos_sz)
    {
        core_relos_sz += sizeof(__u32); /* record size prefix */
        total_sz += core_relos_sz;
    }

    cur = data = calloc(1, total_sz);
    if (!data)
        return -ENOMEM;

    hdr = cur;
    hdr->magic = BTF_MAGIC;
    hdr->version = BTF_VERSION;
    hdr->flags = 0;
    hdr->hdr_len = sizeof(struct btf_ext_header);
    cur += sizeof(struct btf_ext_header);

    /* All offsets are in bytes relative to the end of this header */
    hdr->func_info_off = 0;
    hdr->func_info_len = funcs_sz;
    hdr->line_info_off = funcs_sz;
    hdr->line_info_len = lines_sz;
    hdr->core_relo_off = funcs_sz + lines_sz;
    hdr->core_relo_len = core_relos_sz;

    if (funcs_sz)
    {
        *(__u32 *)cur = func_rec_sz;
        cur += sizeof(__u32);

        for (i = 1; i < linker->sec_cnt; i++)
        {
            struct dst_sec *sec = &linker->secs[i];

            sz = emit_btf_ext_data(linker, cur, sec->sec_name, &sec->func_info);
            if (sz < 0)
            {
                err = sz;
                goto out;
            }

            cur += sz;
        }
    }

    if (lines_sz)
    {
        *(__u32 *)cur = line_rec_sz;
        cur += sizeof(__u32);

        for (i = 1; i < linker->sec_cnt; i++)
        {
            struct dst_sec *sec = &linker->secs[i];

            sz = emit_btf_ext_data(linker, cur, sec->sec_name, &sec->line_info);
            if (sz < 0)
            {
                err = sz;
                goto out;
            }

            cur += sz;
        }
    }

    if (core_relos_sz)
    {
        *(__u32 *)cur = core_relo_rec_sz;
        cur += sizeof(__u32);

        for (i = 1; i < linker->sec_cnt; i++)
        {
            struct dst_sec *sec = &linker->secs[i];

            sz = emit_btf_ext_data(linker, cur, sec->sec_name, &sec->core_relo_info);
            if (sz < 0)
            {
                err = sz;
                goto out;
            }

            cur += sz;
        }
    }

    linker->btf_ext = btf_ext__new(data, total_sz);
    err = libbpf_get_error(linker->btf_ext);
    if (err)
    {
        linker->btf_ext = NULL;
        pr_warn("failed to parse final .BTF.ext data: %d\n", err);
        goto out;
    }

out:
    free(data);
    return err;
}